On a boat having an inboard motor that turns a shaft attached to an external propeller, the shaft passes though a stuffing box, also called a "packing box" or "stern gland". The stuffing box prevents sea water from entering the boat's hull. In many small fiberglass boats, for example, the stuffing box is mounted inboard near the point the shaft exits the hull. The "box" is a cylindrical assembly, typically of bronze, comprising a sleeve threaded on one end to accept adjusting and locking nuts.